Joint custody following divorce is widespread, but implementation is costly when individuals live in different states and so affects interstate mobility. Migration of separated fathers has fallen significantly more than married fathers. We show the causal effect of joint custody using two strategies. First, we survey separated parents to elicit beliefs about the likelihood of interstate moves. Second, we use the staggered adoption of joint custody laws across US states, and show a reduction in actual migration of 11 percentage points for fathers. For mothers, there is no impact on mobility but suggestive evidence of beneficial labor market outcomes.